Ru Dissolution Research Articles

(Energy Technology Division Supramaniam Srinivasan Young Investigator Award Address) Enhanced Oxygen Electrocatalysis By Means of Electronic and Geometric Effects

The slow kinetics of the oxygen reduction and evolution reactions (ORR and OER) impede the widespread uptake of polymer electrolyte membrane (PEM) fuel cells and electrolysers. In order to improve the kinetics of the ORR and reduce the Pt loading at the fuel-cell cathode, we need to enhance the activity, stability and selectivity of the catalyst. This can be achieved by alteration of the electronic properties of the Pt surface atoms (electronic effects [1,2]) and/or modification of the geometric structure (atomic ensemble effects [3]). First, I will address atomic ensemble effects in electrocatalysis. We used a self-ordered molecular pattern of cyanide on Pt(111) surfaces to study the role of atomic ensembles for the ORR [3]. Cyanide groups form an ordered structure that blocks all the three-fold hollow sites, effectively blocking the adsorption of spectator anions in the electrolyte. Nonetheless, the holes in this structure are sufficiently large to allow the adsorption of reaction oxygen molecules. As a consequence, cyanide-modified Pt(111) presents a 25-fold enhancement over Pt(111) [3]. Secondly, I will focus on electronic effects by alloying Pt with other metals. In order to reduce the Pt loading, researchers have intensively studied alloys of Pt with late transition metals such as Ni and Co. However, these compounds typically degrade under fuel-cell conditions, due to dealloying. In contrast, Pt-lanthanide alloys present a very negative alloying energy, which should increase their resistance to degradation. We have studied novel Pt-lanthanide and Pt-alkaline earth electrocatalysts for the ORR. Pt-lanthanide and Pt-alkaline earth alloys are amongst the most active polycrystalline Pt-based catalysts ever reported [1,2], presenting up to a 6-fold increase in ORR activity, relative to Pt. The active phase consists of a compressed Pt overlayer of few Pt layers formed onto the bulk alloys by acid leaching. Notably, the oxygen reduction activity versus the bulk lattice parameter follows a volcano relation [1]. We use the lanthanide contraction to control strain effects and tailor the activity, stability and reactivity of Pt alloys. Finally, I will present new approaches to design and develop more efficient and stable OER electrocatalysts. We have explored the OER activity and stability of RuO2 nanoparticulate catalysts. Sub-monolayer amounts of IrOx on top of RuO2 thin films minimise Ru dissolution [4]. The strategy of tailoring the surface of the catalyst with sub-monolayer amounts of a stable oxide may allow us to tune the stability of active OER catalysts for PEM electrolysers.

Dissolution Induced Self-Selective Zn- and Ru-Doped TiO2 Structure for Electrochemical Generation of KClO3

An electrochemical approach towards synthesis of highly active, stable and surface selective towards chlorine evolution reaction (CER) of (Zn, Ru)-doped TiO2 (Ru0.26Ti0.73Zn0.01Ox) for the purpose of electrochemical generation of KClO3 is demonstrated. The essential ingredients of this approach consists of (1) co-electrodeposition of Ru, Ti and Zn as metallic alloy and subsequent annealing to oxide (2) dissolution under electrochemically oxidative acidic environment of Ru and Zn-rich surface clusters having high selectivity towards oxygen evolution reaction (OER) leaving Ti-enriched structure that is more selective to CER and (3) using this electrocatalyst for KClO3 production under near-neutral pH conditions. Dissolution of Ru- and Zn enriched surfaces result in higher porosity and surface roughness. Furthermore, this electrochemical dissolution of (Zn, Ru)-rich surface results in electrocatalytic structure that is self-selective towards KClO3 formation with higher specific activity.

Electrochemically Activated Iridium Oxide Black as Promising Electrocatalyst Having High Activity and Stability for Oxygen Evolution Reaction

In this study, we report a promising approach to greatly enhance the activity of iridium while retaining its high stability. In the approach, electrochemically activated Ir oxide black (A-IrOx-B) is prepared by the selective etching of a less noble component from the binary oxide. Binary IrOs oxide black (IrOsOx-B) was formed by using a solvent-free method, and then an electrochemical activation was performed by the selective etching of Os oxide in order to obtain the activated Ir oxide black possessing high surface area. Interestingly, it was revealed that the oxygen evolution reaction (OER) activity of A-IrOx-B surpassed that of IrOx black and approached that of the state-of-the-art surface-segregated IrRu oxide (S-IrRuOx-B). Most notably, the OER activity of S-IrRuOx-B decreased after stability testing because of the dissolution of Ru, while the activity of A-IrOx-B changed little and eventually surpassed the activity of S-IrRuOx-B.

Improvement in stability of PtRu electrocatalyst by carbonization of in-situ polymerized polyaniline

As well known, ruthenium is electrochemically unstable in direct methanol fuel cell (DMFC) operation shortening lifetime and deteriorating performance of DMFC device. In this work, a facile methodology for improvement in stability of PtRu electrocatalyst is described, in which PtRu alloyed nanoparticles are decorated by nitrogen-doped carbon (NxC) originating from the carburization of in-situ polymerized polyaniline (PANI). Deceleration in Ru dissolution of NxC protected PtRu electrocatalyst comes from the ionized Ru atoms induced by the carburization of PANI transferring electrons from Ru to nitrogen atoms evidenced by XPS measurement, which results in higher CO tolerance during potential sweeping from 0.6 to 1.0 V versus RHE for NxC decorated PtRu electrocatalyst. Meanwhile, NxC protected PtRu electrocatalyst only losses 10% of active sites after stability estimation; in contrast, 50% of active sites are lost for bare PtRu electrocatalyst. Moreover, fuel cell test illustrates that NxC protected PtRu electrocatalyst shows comparable performance to bare PtRu electrocatalyst.

PtRu nanoparticles embedded in nitrogen doped carbon with highly stable CO tolerance and durability

As is well known, the lower durability and sluggish methanol oxidation reaction (MOR) of PtRu alloy electrocatalyst blocks the commercialization of direct methanol fuel cells (DMFCs). Here, we design a new PtRu electrocatalyst, with highly stable CO tolerance and durability, in which the PtRu nanoparticles are embedded in nitrogen doped carbon layers derived from carbonization of poly(vinyl pyrrolidone). The newly fabricated electrocatalyst exhibits no loss in electrochemical surface area (ECSA) and MOR activity after potential cycling from 0.6–1.0 V versus reversible hydrogen electrode, while commercial CB/PtRu retains only 50% of its initial ECSA. Meanwhile, due to the same protective layers, the Ru dissolution is decelerated, resulting in stable CO tolerance. Methanol oxidation reaction (MOR) testing indicates that the activity of newly fabricated electrocatalyst is two times higher than that of commercial CB/PtRu, and the fuel cell performance of the embedded PtRu electrocatalyst was comparable to that of commercial CB/PtRu. The embedded PtRu electrocatalyst is applicable in real DMFC operation. This study offers important and useful information for the design and fabrication of durable and CO tolerant electrocatalysts.

Importance of Surface IrOx in Stabilizing RuO2 for Oxygen Evolution.

The high precious metal loading and high overpotential of the oxygen evolution reaction (OER) prevents the widespread utilization of polymer electrolyte membrane (PEM) water electrolyzers. Herein we explore the OER activity and stability in acidic electrolyte of a combined IrOx/RuO2 system consisting of RuO2 thin films with submonolayer (1, 2, and 4 Å) amounts of IrOx deposited on top. Operando extended X-ray absorption fine structure (EXAFS) on the Ir L-3 edge revealed a rutile type IrO2 structure with some Ir sites occupied by Ru, IrOx being at the surface of the RuO2 thin film. We monitor corrosion on IrOx/RuO2 thin films by combining electrochemical quartz crystal microbalance (EQCM) with inductively coupled mass spectrometry (ICP-MS). We elucidate the importance of submonolayer surface IrOx in minimizing Ru dissolution. Our work shows that we can tune the surface properties of active OER catalysts, such as RuO2, aiming to achieve higher electrocatalytic stability in PEM electrolyzers.

High stability and performance of PtRu electrocatalyst derived from double polymer coatings

In this work, we focus on the durability, CO tolerance and sluggish methanol oxidation reaction (MOR) of PtRu electrocatalyst. Here, we fabricate a new PtRu electrocatalyst supported on carbon black (CB), in which the PtRu nanoparticles are structurally sandwiched by poly(vinyl pyrrolidone) (PVP) and poly(2,5-benzimidazole) (ABPBI). Due to the first coating (PVP) on CB, the micropores on CB are capped resulting in high Pt utilization efficiency (94%) and catalytic activity toward methanol oxidation. Meanwhile, the second coating (ABPBI) on PtRu nanoparticles fabricated via in-situ polymerization improves the PtRu stability and decelerates the Ru dissolution leading to stable electrochemical surface area (ECSA), MOR activity and CO tolerance during the durability test. Without ABPBI coating, ECSA and catalytic activity remain only 50% after potential cycling. Additionally, the fuel cell performance of double polymer coated electrocatalyst reaches 103 mW cm−2, which is among the highest values reported in the recent literatures.

Stabilization of PtRu electrocatalyst by nitrogen doped carbon layer derived from carbonization of poly(vinyl pyrrolidone)

Improvements on durability and CO tolerance of the electrocatalyst are crucial for widespread commercialization of direct methanol fuel cells (DMFCs). In this work, we describe a new method to stabilize the PtRu electrocatalyst, in which the PtRu nanoparticles are coated by nitrogen doped carbon layer derived from the carbonization of poly(vinyl pyrrolidone). The coated electrocatalyst shows stable electrochemical surface area (ECSA) and methanol oxidation reaction (MOR) activity after 4200 potential cycles from 0.6 V to 1.0 V vs. RHE; while the non-coated and commercial electrocatalyst lose almost 50% of initial ECSA and MOR activity. Meanwhile, the coated electrocatalyst shows twice higher CO tolerance before and after durability test due to the deceleration of the Ru dissolution proved by the XPS measurement after durability test. The mechanism of the stabilization of Ru was the electron delocalization of Ru caused by the carbonization process of PVP, which changes the electronic structure of Ru and makes Ru difficult to be dissolved. The maximum power density of the coated electrocatalyst is 1.7 times higher than that of commercial CB/PtRu, suggesting the coated electrocatalyst is suitable for real DMFC application. The demonstrated method could be easily extended to obtain extraordinary durability of other electrocatalysts.

Development of methods for concentration and dissolution of Rh and Ru from copper slime

In copper (Cu) smelters, precious metals in recycled materials are distributed into electrolytic Cu slime. During the precious metal refining process, rhodium (Rh) and ruthenium (Ru) are concentrated from the slime into residues containing selenium (Se) and tellurium (Te). In this study, new methods were developed for refining Rh and Ru by obtaining aqueous solutions of Rh and Ru and separating Se and Te using a hydrometallurgical process followed by a pyrometallurgical process. In the hydrometallurgical process, Se and Te are dissolved by air oxidation in a sodium hydroxide (NaOH) solution, and Cu is dissolved using diluted sulfuric acid (H2SO4). Se and Te are removed by dissolution and separated from the undissolved Rh and Ru. In the pyrometallurgical process, the residual Se and Te in the Rh and Ru concentrates obtained from the hydrometallurgical process are chlorinated and separated by volatilization. Rh, Ru, Se, and Te are chlorinated by heating to 1000–1100K in chlorine (Cl2) gas. Volatilization was used for separation of Se and Te from Rh and Ru because the vapor pressures of Se and Te chlorides are higher than those of Rh and Ru chlorides. Because the chlorides of Rh and Ru have poor solubility, they are mixed with sodium chloride (NaCl), heated, and converted to soluble complex salts containing Rh and Ru, which are later dissolved in water to obtain an aqueous solution containing concentrated Rh and Ru.

Electrochemical dissolution of Тс–Ru alloys in nitric acid solutions

Electrochemical dissolution of Тс–19 wt % Ru, Тс–45 wt % Ru, and Тс–70 wt % Ru in 1 to 6 M HNO3 solutions in the amperostatic mode was studied. In the solutions formed from anodic dissolution of Тс–Ru alloys, Ru is present in the form of Ru(IV), and Тс, in the form of Тс(VII). A kinetic study of the electrochemical dissolution of the Тс–70 wt % Ru alloy in the examined interval of HNO3 concentrations showed that the alloy dissolved congruently and the Тс(VII) and Ru(IV) accumulation in the solution linearly depended on time. Anodic dissolution of Tc–Ru alloys with lower Ru content was also characterized by a linear increase in the Тс(VII) concentration in the solution with time. On the other hand, formation of poorly soluble hydrated Ru(IV) oxide on the electrode surface was observed simultaneously with accumulation of soluble Ru(IV) species in solution. The rate of electrochemical dissolution of the Тс–70 wt % Ru alloy linearly increased with increasing HNO3 concentration. The rates of electrochemical dissolution of Tc, determined for alloys with lower Ru content, were independent of the HNO3 concentration in the electrolyte. Oxidation of water with the evolution of oxygen on the surface of Tc–Ru alloys was observed simultaneously with the anodic dissolution of Tc and Ru. This process leads to a decrease in the current efficiency of the Tc and Ru dissolution. Examination of the corrosion damage of the working electrode surfaces by scanning electron microscopy shows that the electrochemical dissolution of Tc–Ru alloys leads to uniform corrosion of their surface.

Hydrogen Oxidation Reaction Activity of RuO2 Nanosheets Modified Pt−Ru / C

Carbon supported Pt-Ru systems such as PtRu/C and Pt2Ru3/C have been investigated as anode catalysts of polymer electrolyte membrane fuel cells. However, hydrogen oxidation reaction (HOR) activity for Pt-Ru/C is significantly degraded due to adsorption of carbon monoxide (CO) on the active Pt sites and dissolution of Ru. The addition of metal oxide nanosheets enables to enhance the performance of catalysts. We have reported that RuO2 nanosheets [1] as an additive to Pt/C (RuO2ns-Pt/C) and PtRu/C (RuO2ns-PtRu/C) enhances CO tolerance at room temperature [2]. The HOR current of RuO2ns-Pt/C was improved by 25% compared to Pt/C. RuO2ns-PtRu/C had higher HOR activity and enhanced CO tolerance. Thus, CO poisoning is suppressed by the addition of RuO2 nanosheets. However, it is necessary to measure the performances at higher temperature near actual operating of PEFC. In this study, we report the addition effect of RuO2 nanosheets to PtRu/C and Pt2Ru3/C at 25◦C as well as 70◦C. Chronoamperograms (CA) of commercial and RuO2 nanosheests modified catalysts at 25 and 70◦C are shown in Figures A and B. At 25◦C (Fig. A), the addition of RuO2 nanosheets increased the initial HOR activity and the activity after accelerated durability tests (ADT) in 300 ppm CO containing H2 (CO/H2) as well as pure H2. The current was enhanced in the entire range of CA. The addition of RuO2 nanosheets slightly enhanced CO tolerance and durability which were represented by ratio of density current j (in H2) divided by j (initial in CO/H2) and j (initial in CO/H2) divided by j (after ADT in CO/H2), respectively. This behavior is similar to the previous result of comparing RuO2ns-PtRu/C and PtRu/C [2]. The HOR current at 70◦C showed the different behavior compared to 25◦C (Fig. B). At 70◦C, there was a difference of HOR current between composite and commercial catalysts in CO/H2 while HOR activity of the catalysts was almost the same in pure H2. Thus, the decline in HOR current of composite catalysts was slower than non-modified catalysts. Therefore, the composite catalysts suppresses CO adsorption. In particular, the durability of RuO2ns-Pt2Ru3/C at 70◦C shows higher (93%) than the other catalysts (78~86%). This work was supported in part by the “Polymer Electrolyte Fuel Cell Program” from the New Energy and Industrial Technology Development Organization (NEDO), Japan [1] W. Sugimoto, H. Iwata, Y. Yasunaga, Y. Murakami, and Y. Takasu, Angew. Chem. Int. Ed., 42, 4092 (2003). [2] D. Takimoto, T. Ohnishi, and W. Sugimoto, ECS Electrochem. Lett., 4(5), F35-F37 (2015). Figure 1

Ru-Core@Pt-Shell Nanosheet Anode Catalyst: High Activity and Durability for Hydrogen Oxidation Reaction in the Presence of CO

PtRu alloy nanoparticles are presently used as the anode catalyst for residential polymer electrolyte fuel cells due to its high carbon monoxide (CO) tolerance. Unfortunately, PtRu alloy is not as stable as pure Pt, and the gradual dissolution of Ru from the catalyst surface leads to catalyst degradation. In an earlier study, we have shown that the addition of metal or metal oxide nanosheets as co-catalysts to carbon supported PtRu/C catalysts can enhance the CO tolerance and durability 1,2. Here we describe the synthesis of Ru-core@Pt-shell nanosheets via surface limited redox replacement 3. The CO tolerance and durability of Ru-core@Pt-shell nanosheets towards the hydrogen oxidation reaction activity in the presence of CO is presented. Metallic Ru nanosheets supported on carbon composite was prepared via thermal reduction 2. Pt shell was successively formed on metallic Ru nanosheets via galvanic displacement reaction between Cu and Pt2+ by repeating the displacement reaction twice. The electrochemically active PtRu surface area of Ru-core@Pt-shell nanosheets with 1.5 monolayer Pt-shell (Ru@Pt-1.5ML(ns)/C) was 95 m2 (g-PtRu)-1, and the hydrogen oxidation reaction activity in pure H2 of Ru@Pt-1.5ML(ns)/C was 2.4 times higher than benchmark commercial PtRu/C. The H2 oxidation activity in the presence of 300 ppm CO for core-shell nanosheets was 1.5 times higher than that of PtRu/C. In addition, the activity of Ru@Pt-1.5ML(ns)/C after durability test (0.05 (3sec)-0.40 V (3 sec) for 1000 cycles) was 2.0 and 2.3 times higher than that of PtRu/C and Pt/C, respectively. The high activity in pure H2 could be attributed to the large surface area of core-shell structure, and the enhanced CO tolerance could be attributed to the electronic effect from the Ru nanosheet core. Ru nanosheet core was completely covered with Pt-shell, thus, the degradation of activity due to dissolution of Ru was suppressed. This research was supported in part by the “Polymer Electrolyte Fuel Cell Program” from the New Energy and Industrial Technology Development Organization (NEDO) of Japan. 1) D. Takimoto, T. Ohnishi, and W. Sugimoto, ECS Electrochem. Lett., 4, F35 (2015). 2) D. Takimoto, T. Ohnishi, Y. Ayato, D. Mochizuki, and W. Sugimoto, J. Electrochem. Soc., 163, F367 (2016). 3) J. Nutariya, M. Fayette, N. Dimitrov, and N. Vasiljevic, Electrochim. Acta, 112, 813 (2013). Figure 1

Potentiodynamic dissolution study of PtRu/C electrocatalyst in the presence of methanol

Dissolution of PtRu alloy nanoparticles on carbon support in acidic environment is qualitatively and quantitatively investigated using an advanced electrochemical technique: electrochemical flow cell (EFC) on-line coupled to inductively coupled plasma mass spectrometer (ICP-MS). Profound insights into potential-resolved corrosion stability of Ru-Pt alloy in the potential window from 0.05 to 1.6V vs RHE are obtained. Compared to pure Pt the PtRu alloy is less stable due to destabilisation of Pt surface caused by Ru dissolution together with the presence of weaker Ru-Pt bonds. However, at high potentials (1.5 and 1.6V) Pt gets stabilised, presumably via a Ru catalysed oxygen evolution reaction that acts as a cathodic protection. Both Ru and Pt dissolution are enhanced in the presence of methanol due to consumption of surface ruthenium oxides, presumably Ru2O3, by the methanol oxidation reaction and an inhibited redeposition of Pt ions due to surface blockage of CO. The results may be used as useful guidelines in designing future DMFC and PEMFC catalysts.

Effect of nitrogen-doped PtRu/graphene catalyst on its activity and durability for methanol oxidation

Pt–Ru catalysts are highly regarded and investigated by many investigators. However, how to reduce Ru dissolution remains a major concern for their applications under realistic operating conditions. In this study, nitrogen-doped graphene supporting PtRu catalyst was synthesized by a mild microwave-assisted ethanol reduction process, which exhibited superior electrocatalytic properties for methanol oxidation. The morphologies and microstructures of the as-prepared catalysts were characterized by transmission electron microscopy and X-ray photoelectron spectroscopy. Long-term catalytic stability was evaluated by the accelerated aging cyclic voltammetry test. The results reveal that N-doping is beneficial for the minimization of metal migration and coalescence, and meanwhile it slows Ru dissolution.

On the Origin of the Improved Ruthenium Stability in RuO2–IrO2 Mixed Oxides

High oxygen evolution reaction activity of ruthenium and long term stability of iridium in acidic electrolytes make their mixed oxides attractive candidates for utilization as anodes in water electrolyzers. Indeed, such materials were addressed in numerous previous studies. The application of a scanning flow cell connected to an inductively coupled plasma mass spectrometer allowed us now to examine the stability and activity toward oxygen evolution reaction of such mixed oxides in parallel. The whole composition range of Ir–Ru mixtures has been covered in a thin film material library. In the whole composition range the rate of Ru dissolution is observed to be much higher than that of Ir. Eventually, due to the loss of Ru, the activity of the mixed oxides approaches the value corresponding to pure IrO2. Interestingly, the loss of only a few percent of a monolayer in Ru surface concentration results in a significant drop in activity. Several explanations of this phenomenon are discussed. It is concluded that the herein observed stability of mixed Ir–Ru oxide systems is most likely a result of high corrosion resistance of the iridium component, but not due to an alteration of the material's electronic structure.

Activity and stability of the oxygen evolution reaction on electrodeposited Ru and its thermal oxides

The activity and stability of Ru metal and its thermal oxide films for the oxygen evolution reaction (OER) were investigated. The metallic Ru films were prepared by electrodeposition on a Ti substrate and then thermally oxidized at various temperatures under atmospheric conditions. During long-term operation of the OER with cyclic voltammetry (CV) in H2SO4 electrolyte, changes in the properties of the Ru and its thermal oxides were monitored in terms of their morphology, crystal structure, and electronic structure. In the initial stages of the OER, all of the Ru thermal oxide films underwent an activation process that was related to the continuous removal of low-activity Ru oxides from the surface. With further cycling, the OER activity decreased. The rate of decrease was different for each Ru film and was related to the annealing temperatures. Monitoring of material properties indicates that the amount of stable anhydrous RuO2 is important for OER stability because it prevents both the severe dissolution of metallic Ru beneath the oxide surface and the formation of a less active hydrous RuO2 at the surface.

Properties of Pd–Ru–Rh electrodeposits studied by electrochemical, structural and spectroscopic methods

Abstract Pd–Ru–Rh electrodeposits were obtained potentiostatically from aqueous baths containing different concentrations of Pd, Ru and Rh ions. The formation of Pd–Ru–Rh alloys was confirmed electrochemically through the examination of sample properties in the acidic solution with the use of cyclic voltammetry. The alloy formation was also proved with the use of energy dispersive X-ray spectroscopy and X-ray diffraction. XRD and EDX measurements have certified the homogeneity of a single bulk alloy phase. The XRD results show that the lattice parameter of the Pd–Ru–Rh alloy is smaller than that of Pd, i.e. Pd, Ru and Rh form contracted alloys. This is also mirrored in the course of hydrogen absorption curves (H/M vs. E), with the α–β phase transition region shifted into lower potentials. Pd–Ru–Rh alloys with ca. 98.5% Pd in the bulk absorb by ca. 11% more hydrogen than pure Pd. In the process of electrochemical aging, the surface and bulk of Pd–Ru–Rh electrodes become enriched with Pd due to the preferential electrochemical dissolution of Ru and Rh.

Activity and Stability of Molybdenum-Containing Dispersed Pt Catalysts for CO Tolerance in Proton Exchange Membrane Fuel Cell Anodes

In this work, results of attempts to improve the activity and stability of Mo-containing dispersed Pt catalysts for the CO tolerance in proton exchange membrane fuel cell (PEMFC) anodes are revised for the following approaches: (1) application of a heat treatment at various temperatures ranging from 400 to 700 °C on carbon supported PtMo (60:40, Pt:Mo) electrocatalyst; (2) deposition of Pt and PtMo nanoparticles on carbon-supported molybdenum carbides (Mo2C/C) and; (3) employing ternary and quaternary materials formed by PtMoFe/C, PtMoRu/C and PtMoRuFe/C. The Pt-Mo/C catalyst heat-treated at 600 °C showed higher hydrogen oxidation activity in the absence and in the presence of CO and better stability, compared to all other Mo-containing catalysts. Similar CO tolerances were observed for PtMoRuFe, PtMoFe, PtMoRu supported on carbon, and for Pt supported on Mo2C/C, which also presented better stability, as compared to as-prepared PtMo supported on carbon. However, partial dissolution of Mo, Ru, and Fe from the anodes and their migration toward cathodes during the cell operation was observed.

Enhanced CO Tolerance and Durability of PtRu/C By the Modification with Metallic Ruthenium Nanosheets

PtRu/C is presently used as the anode catalyst for residential polymer electrolyte fuel cells. Anode catalysts with increased CO tolerance are desired since trace amounts of CO residing in reformates poisons the catalysts. In addition, the loss of performance of the catalyst due to gradual dissolution of Ru is known to occur due to start/stop cycles. Thus efforts have been made to develop CO tolerant electrocatalysts with high durability. In an earlier study, the authors showed that the addition of oxide nanosheets to carbon supported PtRu/C catalysts can enhance the CO tolerance and durability 1). Here we have studied catalyst activity and durability of PtRu/C catalysts modified with Ru metal nanosheets towards the hydrogen oxidation reaction in the presence of CO. RuO2 nanosheet was derived via exfoliation of layered H0.2RuO2.1×nH2O through a process reported previously 2,3). The RuO2 nanosheet modified PtRu/C catalyst (RuO2ns-PtRu/C) was synthesized following our previous recipe 1). Heating the RuO2ns-PtRu/C under gas-flow-controlled condition (10% H2 + 90% N2, 250 ml min-1) at 150°C at a rate of 5oC min-1 for 2 hours yielded the modified catalyst with Ru metal nanosheets (Ru metal ns-PtRu/C). The catalyst morphology was characterized by the high-resolution scanning electron microscopy (HR-SEM; Hitachi, S-5000, accelerated voltage of 20 kV) and the transmission electron microscopy (TEM; JEOL, JEM-2010, accelerated voltage of 200 kV). Electrochemical measurements were conducted with a rotating disk electrode (Nikko Keisoku). The working electrode was prepared by depositing 5.5 mg-carbon cm‒2 of the catalyst ink on a mirror-polished glassy carbon rod (6 mm in diameter) and vacuum dried at 60°C for 30 min. A carbon fiber (Toho Tenax Co., HTA-3K) was used as a counter electrode, and a reversible hydrogen electrode (RHE) was used as a reference electrode. Catalytic activity was measured by the chronoamperometry at 20 mV vs. RHE in 0.1 M HClO4 (25oC) with 300 ppm CO/H2. Accelerated durability test (ADT) in 300 ppm CO/H2was conducted by 3000 square-wave potential stepping between 0.05 and 0.4 V vs. RHE with a holding time of 3 sec at each potential. The Ru metal nanosheets remain a lateral size ranging from submicrometer to micrometer of the RuO2 nanosheets. The binding energy of the Ru 3p peak for Ru metal ns-PtRu/C is lower compared to PtRu/C and RuO2ns-PtRu/C, indicating that the RuO2 nanosheets successively metallized. The metallic Ru nanosheets have poor hydrogen oxidation reaction (HOR) activity. The HOR current at 0min of Ru metal ns-PtRu/C was comparable to Pt2Ru3/C, indicating that the metallic Ru nanosheets do not inhibit the H2 diffusion. HOR current of all catalyst gradually decreases due to adsorption of CO on the catalyst surface. The HOR activity in CO/H2 at 20 mV vs. RHE (quasi-steady state current after 60 min) was PtRu/C ≈ Pt2Ru3/C << Ru metal ns-PtRu/C. This indicates that the Ru metal ns-PtRu/C is hardly poisoned by CO. The durability of Ru metal ns-PtRu/C after ADT of 3000 potential steps was higher compared to Pt2Ru3/C, PtRu/C and RuO2ns-PtRu/C. The surface Ru composition of PtRu/C and Ru metal ns-PtRu/C was not change by the durability test, suggesting that the degradation of catalytic activity for PtRu/C and Ru metal ns-PtRu/C is not due to dissolution of Ru atoms. Enhanced of the durability of PtRu/C is attributed to improved CO tolerance by the modification with Ru metal nanosheets. This work was supported in part by the “Polymer Electrolyte Fuel Cell Program” from the New Energy and Industrial Technology Development Organization (NEDO), Japan. 1. D. Takimoto, T. Ohnishi, and W. Sugimoto, ECS Electrochem. Lett., 4, F35 (2015). 2. W. Sugimoto, H. Iwata, Y. Yasunaga, Y. Murakami, and Y. Takasu, Angew. Chem. Int. Ed., 42, 4092 (2003). 3. K. Fukuda, H. Kato, J. Sato, W. Sugimoto, and Y. Takasu, J. Solid State Chem., 182, 2997 (2009).

Structural and Chemical Analysis By Transmission Electron Microscopy of Pt-Ru Membrane Precipitates in Proton Exchange Membrane Fuel Cell Aged Under Reformate

Carbon supported platinum-ruthenium (Pt-Ru/C) nanoparticles are used as anode catalysts in PEMFCs operated under reformate owing to their good carbon monoxide tolerance. The stability of these catalysts during fuel cell operation is still not well known. In this work, we have studied by transmission electron microscopy (TEM) the microstructural evolution of a membrane/electrode assembly after a 1,000 h ageing test under reformate (26 ppm of CO) representative of a stationary application. The analyses clearly show evidence of Ru dissolution from the anode with the observation of Pt-Ru precipitates in the membrane and of small Ru precipitates in some areas of the microporous layer near the anode. This Ru dissolution happens despite a rather low estimated anode overpotential (around 0.2V). The distribution of the Pt-Ru membrane precipitates was found to be similar to that observed in aged MEAs with pure Pt catalysts, i.e. a precipitation band several micrometres away from the cathode catalyst layer made of the largest precipitates along with smaller precipitates between the band and the anode catalyst layer. The structure and the chemistry of the membrane precipitates were accurately analysed 1. The high resolution TEM images and EDS (Energy Dispersive X-Ray Spectroscopy) Pt, Ru elemental maps show that the largest precipitates display a singular flower shape consisting of a Pt-rich face-centred cubic ( fcc) crystallographic structure core surrounded by a Ru-rich hcp (hexagonal close-packed) crystallographic structure shell (Fig. 1 a and b). These results suggest a precipitate formation mechanism (Fig. 1c) where the Ru reduction is catalysed by Pt within the membrane. Moreover, the localization of the precipitation band near the cathode seems to indicate that the Pt in the precipitates comes from the dissolution of cathodic Pt/C and that both Pt and Ru ions are reduced by the hydrogen crossover. In addition, the degradation of a MEA aged in a stack under 10 ppm of CO was studied. The MEA degradation was analysed in different zones of the MEA surface area; near the gas inlet, outlet and in the middle. As the durability tests were applied with continuous registration of current density maps thanks to a S++ ® current scan line device, the observed degradation heterogeneities has been correlated to the local conditions. 1 P. A. Henry, L. Guétaz, N. Pelissier, P.-A. André, S. Escribano, J. Power Sources 275 (2015) 312-321 Aknowledgement : This work was supported by the European Union’s Seventh Framework Program (FP7/2007-2013) for the Fuel Cells and Hydrogen Joint Technology Initiative under grant agreement n°256776 (FCH-JU project Premium Act). Figure 1